Add new vote selection method, vote_recommended

The new vote selection method iterates over all approved delegates,
takes their recommended slates, and combines them to create a maximum-
Merge branch 'develop' into vote_recommended
Instead of failing for invalid delegate recommendations, disapprove the
inept/malicious delegate
Merge branch 'develop' into vote_recommended
Fix build and reflect vote_selection_method properly.
Unmark wallet::select_delegate_vote as const
Merge pull request #559 from BitShares/vote_recommended

Add vote_recommended slate selection method